fix(kyberforge): bridge apm content to Claude Code's flat plugin discovery

Claude Code's (and Copilot's) native plugin installer has zero awareness of
.apm/ nesting -- it convention-scans only flat skills/, agents/, commands/,
hooks.json at each plugin's root. Confirmed via strings on the installed
claude binary and live installs of git@holocron/gitea@holocron/kyberforge@
holocron, all reporting Skills(0) Agents(0) Hooks(0) post ADR-0015's apm
conversion. Root cause (apm_cli/core/plugin_manifest.py): apm's plugin.json
compiler deliberately strips skills/agents/commands keys, assuming the host
already auto-discovers those convention directories -- it has no model of
.apm/ being host-visible at all. Separately, apm's own bundle exporter
(apm_cli/bundle/plugin_exporter.py, behind `apm pack --format plugin`)
implements the correct .apm/ -> flat mapping, but only ever targeted
build/<name>-<version>/, a path nothing in marketplace.json's source: points
at.

scripts/sync-plugin-content.sh wraps that bundle exporter and copies its
agents/, skills/, commands/, instructions/, extensions/, and merged
hooks.json back into each plugin's own root as a second tracked
compiled-output category -- same governance status as
.claude-plugin/plugin.json: generated from .apm/, never hand-edited. tests/
subdirectories are excluded from the mirror (dev fixtures, not host-visible
runtime content; several hardcode a relative repo-root walk-up sized for the
.apm/-nested depth, which breaks when duplicated one level shallower).
Applied for real across all 6 plugins and verified two ways: `claude plugin
validate --strict` passes on every real plugin directory, and a live
`claude --plugin-dir <path> -p "list skills/agents"` behavioral test
confirms content is now actually discovered.

Also, from the same issue #90 review round:
- scripts/check-manifests.sh pointed at each plugin's root-level plugin.json
  (checking skills/hooks/mcpServers/agents pointer fields) -- that file was a
  stale near-duplicate of .claude-plugin/plugin.json nothing else read or
  wrote, now deleted across all 6 plugins. check-manifests.sh is rewritten to
  validate .claude-plugin/plugin.json instead, and drops the pointer-field
  checks entirely (nothing to check -- those fields are correctly absent by
  design). Content-presence drift is now check-plugin-content-sync's job, a
  new pre-push hook wired in .pre-commit-config.yaml.

docs/adr/0017 records the root cause and decision in full, including two
rejected alternatives (patching plugin.json's path fields directly -- apm's
compiler strips them on every run; pointing marketplace.json at apm pack's
build/ output -- a version-suffixed non-source directory nothing can install
from without an extra build step). ADR-0015 and CONTEXT.md are updated to
point at it.

Refs: #90

plugins/git/skills/git-submodules/references/sources.md

@@ -0,0 +1,29 @@
+---
+topic: submodules
+source_keys:
+  - git-scm-submodule-docs
+---
+
+## git-scm-submodule-docs
+
+**Description:** Official git-scm.com reference for `git submodule` — all subcommands, flags, configuration keys, and behaviour details.
+
+**Source:** https://git-scm.com/docs/git-submodule
+
+- **Research doc:** plugins/git/docs/research/docs/git/submodules.md (whole-document reference — the research doc is organized by descriptive prose headings such as "Concept Overview" and "Key Commands" rather than a heading matching this slug; this key covers the entire doc, not a single section)
+
+**Contributing files:**
+- SKILL.md (all sections)
+- references/submodules.md (all sections)
+
+---
+
+Other source keys extracted during the git plugin research phase inform sibling skills in the git workflow suite, not this one:
+
+- `context7-git-htmldocs` — git:branches, git:history, git:remotes
+- `git-scm-docs` — git:configuration
+- `git-scm-worktree-docs` — git:worktrees
+- `nvie-gitflow-post`, `atlassian-gitflow-tutorial`, `gitflow-cheatsheet` — git:branches
+- `conventional-commits-spec`, `commitlint-config-conventional` — git:commits
+- `git-scm-push-docs`, `git-scm-fetch-docs`, `git-scm-pull-docs`, `git-scm-remote-docs` — git:remotes
+- `git-scm-bisect-docs`, `git-scm-log-docs`, `git-scm-diff-docs` — git:history